Spearfish answered the Alfa class's speed

How did NATO answer the Alfa class submarine's very high speed?

Britain developed the much faster Spearfish torpedo, while the Alfa's own sensors became far less useful at top speed.

Spearfish was the principal British torpedo response to the Alfa class's speed of about 42 knots. The quoted declared speed of 72 knots would give the torpedo a substantial advantage, with a faster actual speed suggested as a possibility. The Alfa's own speed also imposed a sensing penalty. A later audience comment describes it as almost blind at 20 knots and unable to detect even noisemaker decoys at 42 knots.